1. Field of the Invention
The present invention relates to a memory device. Further, the present invention relates to a semiconductor device including the memory device.
2. Description of the Related Art
In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor having high mobility and uniform element characteristics, has attracted attention as a material of an active layer of a transistor. Various metal oxides are used for a variety of applications. For example, indium oxide is used as a material of a pixel electrode in a liquid crystal display device. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2).